Positive electrode, method for preparing the same and electrochemical device

ABSTRACT

The present disclosure relates to the technical field of energy storage, and in particular, relates to a positive electrode, a method for preparing the positive electrode and an electrochemical device. The positive electrode includes a current collector and a positive electrode active material layer that contains positive electrode active material and is arranged on at least one surface of the current collector. An inorganic layer having a thickness of 20 nm to 2000 nm is arranged on the surface of the at least one positive electrode active material layer away from the current collector. The inorganic layer is a porous dielectric layer containing no binder, and the inorganic layer has a porosity of 10%˜60%. The positive electrode active material layer according to the present disclosure significantly improves the cycle performance, high-temperature storage performance and safety of the electrochemical device.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Chinese Patent ApplicationNo. CN201711478741.0, filed on Dec. 29, 2017, the content of which isincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present disclosure relates to the technical field of energy storage,and in particular, relates to a positive electrode, a method forpreparing the positive electrode and an electrochemical device.

BACKGROUND

Lithium-ion batteries have become the main energy source for variousconsumer electronics, power products, and energy storages and areapplied in various aspects of daily life, because they have charactersof high working voltage, high energy density, no memory effect,environmental protection and the like. In recent years, the market,especially the power battery market, raises higher requirements on thebattery energy density and charging efficiency. With regards to thesecondary battery, improvement in the battery energy density is mainlyachieved in two manners: i) increasing a cut-off charge voltage of thebattery, and ii) increasing capacity of positive electrode activematerial. However, under an excessively high charge-discharge voltage,lithium-ions may de-intercalate excessively in most of the positiveelectrode active material, and its crystal structure is likely to changeor even collapse. Under a high voltage, organic solvents and additivesin the electrolyte tend to decompose due to their insufficient chemicalstability. Particularly, when the lithium transition metal oxidecontains strong oxidizing nickel ions, it is likely to cause sidereactions in the electrolyte, resulting in a serious gas productionproblem, which further leads to a decrease in the battery capacity, adeterioration of cycle performance and severe degradation ofhigh-temperature storage performance. These not only cause irreversibledamage to the battery life, but also lead to potential safety hazards.

On the other hand, the increase in capacity of the positive activematerial is usually achieved by the use of lithium transition metaloxides with a higher theoretical capacity per gram and an increase incompaction density. However, with the increasing of the compactiondensity of the positive electrode, the electrode porosity decreases,ability of absorbing the electrolyte of the electrode is weakened, ionconductivity of the electrode degrades, thereby resulting in a higheroverall resistance of the battery and a deterioration of the cycleperformance of the battery.

Currently, in order to solve the above problems, positive activematerial particles are coated and modified to stabilize the crystalstructure of the particles and avoid electrolyte side reactions on thesurface of the particles. However, several problems and difficultiesexist in the method, including uneven coating, coating shedding,particle agglomeration, and complex coating processes. In addition,since most of the coating substance of the particles is chemically inertinorganic material, the electrical conductivity of the active materialparticles in the electrode is poor, the DCR of the battery is relativelyhigh, and electrolyte-retaining rate is not obviously improved.

SUMMARY

A first aspect of the present disclosure provides a positive electrode.The electrode includes a current collector, and a positive electrodeactive material layer that is arranged on at least one surface of thecurrent collector and contains positive electrode active material. Theelectrode further includes an inorganic layer. The inorganic layer has athickness of 20 nm to 2000 nm and is arranged on a surface of the atleast one positive electrode active material layer away from the currentcollector. The inorganic layer is a porous dielectric layer containingno binder, and the inorganic layer has a porosity of 10%˜60%.

A second aspect of the present disclosure provides a method forpreparing the positive electrode according to the first aspect. Themethod includes at least following steps: preparing a slurry by mixing apositive electrode active material, a binder and a conductive agent;coating on at least one surface of a current collector with the slurryand drying the slurry to form a positive electrode active materiallayer; and preparing an inorganic layer having a thickness of 20 nm to2000 nm on a surface of the at least one positive electrode activematerial layer away from the current collector by vapor depositionmethod. The inorganic layer is a porous dielectric layer containing nobinder, and the inorganic layer has a porosity of 10%˜60%.

A third aspect of the present disclosure provides an electrochemicaldevice including the above positive electrode.

The technical solutions of the present disclosure have at least thefollowing beneficial effects.

In the present disclosure, the inorganic layer provided on the surfaceof the positive electrode active material layer has high iontransmission rate, good mechanical properties and good chemicalstability, and is stable even under a high voltage. By adopting thepositive electrode provided with such inorganic layer, on the one hand,the surface resistance of the positive electrode can be increased,thereby effectively avoiding short circuit caused by direct contactbetween the positive and negative electrodes under the extreme situationor abuse, thus improving the safety performance of the battery; on theother hand, the above inorganic layer can slow down surface structurechange on active material surface of the positive active material athigh voltage, thereby improving the stability and long-term reliabilityof the positive electrode material, and improving the capacity retentionrate after cycles of the battery. Further, the inorganic layer canstabilize the contact interface between the positive electrode activematerial and the electrolyte, and inhibit the occurrence of sidereactions of the electrolyte on the surface of the active material,thereby effectively improving the problem of gas production during thestorage of the battery. At the same time, in order to meet thehigh-capacity requirement, generally, the compaction density of theternary NCM positive electrode is not lower than 3.4 g/cm³, and thecompaction density of the positive electrode of lithium iron phosphatetype is not lower than 2.2 g/cm³, and high compaction density leads to apoor electrolyte wettability and a low electrolyte retaining rate of theelectrode. By providing the hydrophilic inorganic layer on the surfaceof the electrode, the wettability, the number of channels for iontransmission and electrolyte retaining rate of the electrode are allincreased, thereby guaranteeing a high ion passing rate between theelectrode and the electrolyte.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a structural schematic diagram of a positive electrodeaccording to an embodiment of the present disclosure;

FIG. 2 is a structural schematic diagram of another positive electrodeaccording to an embodiment of the present disclosure;

FIG. 3 is a structural schematic diagram of another positive electrodeaccording to an embodiment of the present disclosure; and

FIG. 4 is a SEM photograph (3000×) of a cross-section of a positiveelectrode according to an embodiment of the present disclosure.

REFERENCE SIGNS

1—current collector;

2—positive electrode active material layer;

3—inorganic layer.

DESCRIPTION OF EMBODIMENTS

The present disclosure is further described below by means ofEmbodiments and Comparative Examples. These Embodiments are merely usedfor illustrating the present disclosure, but not intended to limit thepresent disclosure. Without departing from the scope of the technicalsolutions of the present disclosure, any modification or equivalentsubstitution of the technical solutions of the present disclosure shallfall within the protection scope of the present disclosure.

The present disclosure aims to provide a positive electrode, a methodfor preparing the positive electrode and a lithium-ion battery adoptingthe positive electrode.

A positive electrode according to a first aspect of embodiments of thepresent disclosure is firstly described as below.

The positive electrode according to the first aspect includes a currentcollector and at least one positive electrode active material layercontaining positive electrode active material and arranged on at leastone surface of the current collector. There may be a positive electrodeactive material layer provided on one surface of the current collector,or there may be positive electrode active material layers provided ontwo opposite surfaces of the current collector.

FIGS. 1 to 3 illustrate structural schematic diagrams of positiveelectrode according to the embodiments of the present disclosure. Asshown in FIG. 1, a positive electrode active material layer 2 isprovided on a surface of the current collector 1, and an inorganic layer3 is provided on a surface of the positive electrode active materiallayer 2. As shown in FIGS. 2 and 3, two positive electrode activematerial layers 2 are provided on two surfaces of the current collector1. In the, positive electrode shown in FIG. 2, two inorganic layers 3are provided on the surface of each positive electrode active materiallayer 2 away from the current collector 1. In the positive electrodeshown in FIG. 3, an inorganic layer 3 is provided on a surface of one ofthe two positive electrode active material layers 2.

FIG. 4 shows a scanning electron microscope (SEM) photograph at amagnification of 3000 times of a cross-section of a positive electrodeaccording to an embodiment of the present disclosure. As shown in FIG.4, an inorganic layer having a certain thickness is provided on thesurface of the positive electrode. Pores of the inorganic layer cannotbe clearly distinguished in the photograph due to their pore diameter ofsmaller than 100 nm.

When a cut-off charge voltage of the positive electrode active materialis higher than a normal working voltage of the positive electrodematerial, the crystal structure of the positive electrode activematerial is likely to undergo a structural phase transition, and sidereactions of the electrolyte are likely to occur at the surface of thepositive electrode active material. In this way, the electrochemicaldevice may produce a large amount of gas, which further results in thedeterioration of the cycle performance, battery life and safetyperformance. In view of this, in an embodiment of the presentdisclosure, an inorganic layer having a thickness of 20 nm to 2000 nm isprovided on the surface of at least one positive electrode activematerial layer away from the current collector, and the inorganic layeris an inorganic dielectric layer having a porous structure. In this way,without affecting the energy density of the battery, an ultrathin andporous interface, which is ion permeable and has stable chemicalproperties, is formed between the positive electrode active material andthe electrolyte, thereby enhancing the stability and long-termreliability of the positive electrode active material, Further,structural change of the positive electrode active material as well asthe side reactions occurring between the electrolyte and the surface ofthe positive electrode active material can be avoided in the positiveelectrode under conditions of high voltage, high temperature and fastcharging, thereby improving the cycle life under a high voltage andinhibiting the gas production from electrolyte decomposition.

In the embodiments of the present disclosure, the inorganic layercontains no binder. Thus, the inorganic layer is not connected to thepositive electrode active material by binder, thereby avoiding thedeterioration of the electrical properties and safety problems caused bythe peeling of the inorganic layer in case of non-uniform distributionof the binder, binder failure due to the immersing in the electrolyte,long-term cycle or mechanical abuse. In the meantime, the positiveelectrode active material layer has a smooth surface after a coldpressing, so that the inorganic layer also has a good uniformity on thesurface of the positive electrode active material layer, and thethickness deviation can be controlled to be within ±5%.

The inorganic layer has a porosity of 10%˜60%. An upper limit of theporosity of the inorganic layer is 60%, 55%, 50%, 45%, 40%, or 35%. Alower limit of the porosity of the inorganic layer is 10%, 12%, 15%,18%, 20%, 22%, 25%, or 30%. The porosity of the inorganic layer can bein any range consisting of any value of the above upper limits and anyvalue of the above lower limits. If the porosity of the inorganic layeris lower than 10%, even the surface of the electrode has a goodwettability, a number of channels on the surface of the electrode thatare permeable for the electrolyte and absorbing amount of the channelsare both too little, so that the ion-transmission performance anddynamic performance of the battery are relatively poor. If the porosityof the inorganic layer is higher than 60%, contact area between theelectrolyte and the positive electrode active material is too large, sothat the effect of suppressing the side reactions of the electrolyte isunobvious, and the structure of the inorganic layer is too loose to havegood mechanical properties, thereby affecting the long-term reliabilityof the battery.

A method for measuring the porosity of the inorganic layer is performedas follows: the positive electrode (only including the current collectorand the positive electrode active material layer provided on the surfaceof the current collector) is cut out into two electrode samples with thesame area, one of which is untreated and the other one of which isprepared with an inorganic layer; the two samples mentioned above arerespectively dried in a vacuum oven at 105° C. for 2 h, and then placedin a desiccator for cooling down and then for measuring; the samples arewrapped with A4 papers and flattened on a die, and punched with apunching machine, so as to make the samples to be ready for measuring;the thickness of each sample is firstly measured by a micrometer tocalculate an apparent volume of each sample based on a surface area andthe thickness of each sample, and the apparent volume of the untreatedelectrode sample and the apparent volume of the electrode sampleprepared with the inorganic layer are respectively denoted as V1 and V2;then the real volume of each sample is measured by an AccuPyc IIdensitometer, and the real volume of the untreated electrode sample andthe real volume of the electrode sample prepared with the inorganiclayer are respectively denoted as V3 and V4. It can be conducted thatthe porosity of the inorganic layer is equal to[V2−V4−(V1−V3)]/(V2−V1)×100%.

In other embodiments, the porosity of the inorganic layer is 20% to 50%,preferably 25% to 40%.

As an improvement of the positive electrode according to the embodimentof the present disclosure, the thickness of the inorganic layer is in arange of 50 nm to 1500 nm. An upper limit of the thickness of theinorganic layer can be 1500 nm, 1400 nm, 1300 nm, 1200 nm, 1100 nm, 1000nm, 990 nm, 950 nm, 900 nm, 850 nm, 800 nm, 750 nm, 720 nm, 700 nm, 680nm, 650 nm, 600 nm, 550 nm, or 500 nm. A lower limit of the thickness ofthe inorganic layer can be 50 nm, 55 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100nm, 110 nm, 150 nm, 200 nm, 250 nm, 280 nm, 300 nm, 350 nm, 380 nm, 400nm, 430 nm, 450 nm, or 490 nm. The thickness of the inorganic layer canbe in any range consisting of any value of the above upper limits andany value of the above lower limits. If the inorganic layer is too thin,the interface between the positive electrode active material layer andthe electrolyte cannot be effectively stabilized. If the inorganic layeris too thick, it will have a certain adverse effect on theion-permeability of the electrode, thereby further adversely affectingthe cycle and dynamic performances of the battery adopting thiselectrode, also increasing the thickness of the electrode and reducingthe energy density of the battery.

In other embodiments, the thickness of the inorganic layer is preferablyin a range of 100 nm to 1000 nm, and more preferably 150 nm to 500 nm.Within the above ranges, not only the wettability of the electrode withthe electrolyte and the electrolyte-retaining rate and ion-permeabilityof the electrode can be improved, but also the thickness and mass of theelectrode can only have a small increase, which can facilitate anincrease in the energy density of the battery.

As an improvement of the embodiments of the present disclosure, theinorganic layer is manufactured by using vapor deposition method. Sincethe inorganic layer manufactured by vapor deposition method has a stablestructure, properties and good uniformity, the thickness deviation canbe controlled to be within ±5%. In addition, molecules of the inorganiclayer are bonded to the positive electrode active material layer bychemical bonds, so that the inorganic layer is strongly bonded to thepositive electrode active material layer and unlikely to be peeled off.By using vapor deposition method, the thickness of the inorganic layercan be reduced to a certain extent, and the bonding force between theinorganic layer and the positive electrode active material layer can beincreased.

Preferably, the vapor deposition method includes Atomic Layer Deposition(ALD), Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD)and Thermal Evaporation Deposition. Preferably, plasma assisted thermalevaporation deposition, reactive ion beam sputtering deposition,electron beam evaporation, magnetron sputtering method, or plasma arcplating can be used.

As an improvement of the embodiment of the present disclosure, theinorganic layer spreads on the surface of the positive electrode activematerial layer away from the current collector and inner walls of poreson the surface of the positive electrode active material layer. Sincethe inorganic layer according to the embodiments of the presentdisclosure is relatively thin, the inorganic layer, when manufactured byusing vapor deposition method, not only can be deposited on the surfaceof the positive electrode active material layer, but also can penetratein the electrode and be deposited on the inner walls between the activematerial particles to a certain depth from the surface of the electrodeactive material layer. With an increase of the thickness of theinorganic layer, the coverage rate of the pores between particles in theelectrode active material layer is increased, the effect of suppressingthe side reactions of the electrolyte on the surface of the particles ismore outstanding, and the resistivity of the surface of the electrodeactive material layer is also increased, thereby effectively solving theproblems of gas production and safety in the battery.

As an improvement of the embodiment of the present disclosure,preferably, a ratio of the depth of the inorganic layer spreading in thepores on the surface of the positive electrode active material layer tothe thickness of the positive electrode active material layer is in arange of 1/1000 to 1/10. In a specific preparing process, for example,when using vapor deposition method, the coating depth of the inorganiclayer inside the positive electrode active material layer increases witha deposition thickness. However, if the coating depth is too deep, forexample, greater than 1/10 of the thickness of the positive electrodeactive material layer, the pores of the positive electrode activematerial layer can be blocked by a large amount of the inorganic layer,resulting in a deterioration of the ion transmission performance of thebattery. If the inorganic layer coats the inner walls of the pores to ashallow depth, for example smaller than 1/1000 of the thickness of thepositive electrode active material layer, a large area of the surfacesof active material particles inside the positive electrode activematerial layer are exposed, such that the effect of stabilizing theinterface by the inorganic layer is not obvious under a voltage abovethe normal working voltage.

As an improvement of the embodiment of the present disclosure, theinorganic layer has a porous structure formed by stacked nanoclusters.The nanoclusters are ultrafine particles in nanometer scale. Theultrafine particles in nanometer scale can be formed by agglomeratingseveral to hundreds of atoms and molecules, and these ultrafineparticles are referred to as nanoclusters. Pores between thenanoclusters are used for ion transmission. The pores can have anaverage pore diameter in a range of 0.1 nm to 100 nm. If the averagepore diameter is too small, the ion transmission performance of theelectrode may be affected, thereby resulting in a poor dynamicperformance of the battery adopting the electrode. If the average porediameter is too large, in order to guarantee the above mentionedporosity, the structure of the inorganic layer may be too loose, thecontact area between the electrolyte and the positive electrode activematerial is too large, and the effect of suppressing the side reactionsof the electrolyte is not obvious. Moreover, since the structure of theinorganic layer is likely to be too loose, the mechanical properties ofthe inorganic layer may be poor, and the reliability of the batteringduring a long-term use may be reduced.

As an improvement of the embodiments of the present disclosure, thenanoclusters are nanograins of an inorganic dielectric material havingan average particle size of 2 nm to 500 nm. If the average particle sizeof the nanoclusters is too small, the nanoclusters are stacked tootightly to form the inorganic layer having a specific porosity. If theaverage particle size of the nanoclusters is too large, the nanoclustersare stacked too loose to form the inorganic layer having a specificporosity.

As an improvement of the embodiments of the present disclosure, theinorganic layer has a mass per unit area of 0.03 g/m²-7.0 g/m². The massper unit area of the inorganic layer is dependent on parameters such asthe distribution of the inorganic layer on the electrode active materiallayer, and the thickness, porosity and pore diameter of the inorganiclayer, and the like. Therefore, merely controlling the thickness of theinorganic layer cannot fundamentally guarantee that the active materialon the surface of the positive electrode active material layer can beeffectively insulated from the electrolyte. In the present disclosure,it is found that, by controlling the mass of the inorganic layer, theactive material on the surface of the positive electrode active materiallayer can be effectively insulated from the electrolyte, therebyeffectively guaranteeing the dynamic performance of the battery. Whenthe mass per unit area of the inorganic layer is too small, the surfaceof the positive electrode active material layer has a relatively smallamount of dielectric material, so that the interface between theelectrolyte and the active material particles on the surface of theelectrode active material layer cannot be effectively stabilize, and theproblems of gas production and poor cycle performance still exist. Whenthe mass per unit area of the inorganic layer is too great, thethickness of the inorganic layer is increased and the inorganic layermay block a large amount of pores on the surface of the electrode activematerial layer, so that distance of ion transmission is increased, thenumber of channels for ion transmission is reduced, and intercalation ofions into the positive electrode active material layer andde-intercalation of ions from the positive electrode active materiallayer are both negatively affected, thereby resulting in thedeterioration of the dynamic performance of battery.

The mass per unit area of the inorganic layer is determined by a weightdifference method as follows. Before the determination of parameters forpreparing the inorganic layer with vapor deposition method, a positiveelectrode is taken for adjusting equipment parameters. The positiveelectrode is cut into 20 pieces of 50 mm×50 mm square pieces, and themass of each piece is accurately weighed using a six-digit precisionbalance so as to calculate an average weight used as the mass of acopper foil square piece, which is denoted as m0. After the equipmentparameters are determined, an inorganic layer is deposited on thepositive electrode, and the positive electrode deposited with theinorganic layer is cut into 20 pieces of 50 mm×50 mm square pieces, andthe mass of each piece is weighed so as to calculate an average mass,which is denoted as m1. Therefore, the mass per unit area of theinorganic layer is equal to (m1−m0)/25cm².

As an improvement of the embodiments of the present disclosure, theporosity of the inorganic layer is greater than the porosity of thepositive electrode active material layer. In order to meet thehigh-capacity requirement of the battery, the compaction density ofelectrode needs to be further increased, which can result in that theelectrode has a low inside porosity and low wettability andelectrolyte-retaining rate. By providing on the positive electrodeactive material layer the inorganic layer having a greater porosity thanthe porosity of the positive electrode active material layer, thewettability and electrolyte-retaining rate of the electrode can beimproved, the dynamic performance can be enhanced, and the electrodeactive material can be further isolated from contacting with theelectrolyte, thereby optimizing the stability of the interface of thebattery.

As an improvement of the embodiments of the present disclosure, theinorganic layer contains a material selected from a group consisting ofaluminum oxide, AlO(OH), aluminum nitride, aluminum fluoride, siliconoxide, silicon nitride, silicon fluoride, titanium oxide, titaniumnitride, titanium fluoride, zinc oxide, zinc nitride, zinc fluoride,magnesium oxide, magnesium nitride, magnesium fluoride, zirconium oxide,zirconium nitride, zirconium fluoride, calcium oxide, calcium nitride,calcium fluoride, barium oxide, barium nitride, barium fluoride, orcombinations thereof.

As an improvement of the embodiments of the present disclosure, thepositive electrode active material is selected from a group consistingof Li_(z)Co_(1−(x+y))Ni_(x)Mn_(y)O₂, LiCoO₂, LiNiO₂, LiCo_(a)Ni_(1−a)O₂,LiVPO₄, Li₂MnO₃, LiMn_(2−b)M_(b)O₄, LiFe_(c)Mn_(1−c)PO₄, or combinationsthereof, in which 0<x<1, 0<y<1, 0.9<z<12, 0<a<1, 0≤b<2, 0<c≤1, and M isselected from a group consisting of Li, Al, Mg, Zn, Cr, La, Nd, Ti, Sn,V, or combinations thereof.

As an improvement of the embodiments of the present disclosure, theparticles of the positive electrode active material are provided with acoating layer, and the coating layer contains at least one of Al₂O₃,CaO, SiO₂, TiO₂, B₂O₃, MgO, ZrO₂, or lithium transition metal phosphate.The lithium transition metal phosphate isLi_(x1)M′_(y1)N′_(1−y1)PO_(4−λ)B_(ζ), in which 0.9≤x1≤1.2, 0.6≤y1≤1.0,0≤λ≤0.5, 0≤ζ≤0.5, M′ is selected from a group consisting of Ni, Co, Mn,or combinations thereof, and N′ is selected from a group consisting ofMg, Al, Ti, Ni, Co, Mn, Cu, or combinations thereof.

As an improvement of the embodiments of the present disclosure, thecoating layer has a thickness of 5 nm to 500 nm. If the coating layer ofthe positive electrode active material particles is too thin, thecoating layer cannot cover the active material over a wide range andthus fails to provide a protective effect. If the coating layer of thepositive electrode active material particles is too thick, the energydensity may be negatively affected, the conductivity between the activematerial particles may be reduced, and the internal resistance and DCresistance (DCR) of the battery may be increased.

A method for preparing a positive electrode according to the secondaspect of the present disclosure is described as follows.

The method for preparing a positive electrode according to the secondaspect of the present disclosure incudes at least following steps:preparing a slurry by mixing a positive electrode active material, abinder and a conductive agent; coating on at least one surface of acurrent collector with the slurry and drying the slurry to form apositive electrode active material layer; and preparing an inorganiclayer having a thickness of 20 nm to 2000 nm on a surface of the atleast one positive electrode active material layer away from the currentcollector by vapor deposition method. The inorganic layer is a porousdielectric layer containing no binder, and has a porosity of 10%˜60%.

In an embodiment, the vapor deposition method includes Atomic LayerDeposition (ALD) , Chemical Vapor Deposition (CVD), Physical VaporDeposition (PVD) and Thermal Evaporation Deposition. Preferably, plasmaassisted thermal evaporation deposition, reactive ion beam sputteringdeposition, electron beam evaporation, magnetron sputtering method, orplasma arc plating can be used.

Specifically, the inorganic layer can be deposited by means of a thermalevaporation gas phase reaction method, which includes the followingsteps:

Step (1): evacuating a deposition chamber to a pressure of 0.001 Pa orlower;

Step (2): introducing a reactive gas ‘a’ into the deposition chamber;

Step (3): heating a precursor ‘b’ of the porous inorganic layer in aheating chamber into a gaseous precursor, and introducing the gaseousprecursor into the deposition chamber; and

Step (4): adjusting flow of gas source, substrate temperature, anddeposition distance in the deposition chamber, so that the reactive gas‘a’ chemically reacts with the gaseous precursor ‘b’ to form theinorganic layer on the surface of the positive electrode active materiallayer.

The reactive gas ‘a’ can be selected from a group consisting of oxygen,ozone, carbon dioxide, water vapor, nitrogen, nitric oxide, nitrogendioxide, ammonia, or combinations thereof. The precursor ‘b’ of theinorganic layer can be selected from a group consisting of pure metals,alloys, metal alkyl compounds, metal nitrate compounds, metal acetatecompounds, and metal sulfuric acids of Al, Si, Ti, Mg, Zr, Ca, Zn or Ba,or combinations thereof.

The heating current in Step (3) is in a range of 50 A to 300 A.

In Step (4), the substrate temperature is controlled to be lower than100° C., the volume flow of the reactive gas is 100 sccm-2000 sccm, andplasma can be introduced into the reaction space to increase theactivity of the reactive gas.

The electrochemical device according to the third aspect of the presentdisclosure is described as below.

The electrochemical device of the present disclosure can be alithium-ion secondary battery, a lithium primary battery, a sodium ionbattery, or a magnesium ion battery, but not limited thereto.

The following takes a lithium-ion secondary battery as an example tofurther discuss the embodiments of the present disclosure.

The lithium-ion secondary battery includes a positive electrode, anegative electrode, a separator, and an electrolyte. The positiveelectrode is the positive electrode according to the first aspect of thepresent disclosure.

When the electrochemical device is a lithium-ion secondary battery, theinorganic layer is still stable even the voltage between the positiveand negative electrode is above 5 V.

Embodiments and Comparative Examples with respect to the positiveelectrode, the method for preparing the positive electrode, and thelithium-ion secondary battery are discussed below.

In the embodiments of the present disclosure, the positive electrodeactive material can be a layered lithium transition metal oxide,lithium-rich manganese-based oxide, lithium iron phosphate, lithiumcobaltate, or a positive electrode active material doped or coated withlithium transition metal oxide, lithium-rich manganese-based oxide,lithium iron phosphate, lithium cobaltate. In the embodiments of thepresent disclosure, LiNi_(0.8)Co_(0.1)Mn_(0.1)O₂ (NCM811) is taken as anexample of the layered lithium transition metal oxide. The activematerial of the negative electrode is not specified, and can be selectedfrom a group consisting of natural graphite, artificial graphite, softcarbon, hard carbon, silicon, silicon oxide compound, silicon-carboncomposite, lithium, or combinations thereof. In the embodiments of thepresent disclosure, the natural graphite is taken as an example of theactive material of the negative electrode.

Preparation of Positive Electrode

(1) A positive electrode active material (NCM), a conductive agent ofacetylene black (SP), and a binder of polyvinylidene fluoride (PVDF) aremixed at a weight ratio of 96:2:2. N-methylpyrrolidone is added as asolvent, and mixed to obtain a positive electrode slurry.

(2) The positive electrode slurry is uniformly coated on an aluminumfoil that acts as a current collector, followed by being dried at 85°C., cold-pressed, trimmed, and cut into plates and then into strips, andfinally dried at 85° C. under vacuum for 4 hours to obtain a positiveelectrode.

(3) The inorganic layer is prepared on the surface of at least oneelectrode active material layer of the above positive electrode, forexample, by plasma assisted thermal evaporation deposition, in which theheating source is an electron beam and the heating target is a pureelement (such as Al, Si, Mg, etc.) except for oxygen in the inorganiclayer. Under the vacuum condition, an oxygen-containing active gas(e.g., oxygen, ozone, oxygen ion, nitrogen monoxide, nitrogen dioxide,carbon dioxide, water vapor, etc.) is used as the reactive gas. Thesubstrate temperature is controlled to be lower than 100° C. Byadjusting the heating current (50 A to 300 A), the vacuum degree ofprocess chamber (10⁻¹ Pa to 10⁻³ Pa), the oxygen volume flow (100 sccmto 2000 sccm), the plasma power (300 W to 600 W) and the processingtime, the deposition rate of the inorganic layer on the surface of thepositive electrode can be adjusted, and thus the thickness, composition,and micromorphology of the inorganic layer can be further adjusted.

Preparation of Negative Electrode

A negative electrode active material of artificial graphite, aconductive agent of acetylene black, a binder of styrene butadienerubber (SBR), and a thickener of carboxyl methyl cellulose (CMC) aremixed together at a weight ratio of 96:1:2:1, and deionized water isadded therein as a solvent, evenly mixed to obtain a negative electrodeslurry. The above negative electrode slurry is uniformly coated on acopper foil that acts as a negative current collector, followed by beingdried at 80° C. to 90° C., cold-pressed, trimmed, cut into plates andthen into strips, and finally dried at 110° C. under vacuum for 4 hoursto obtain a negative electrode.

Preparation of Electrolyte

A basic electrolyte is prepared with dimethyl carbonate (DMC), ethylmethyl carbonate (EMC) and ethylene carbonate (EC) at a mass ratio of5:2:3. An electrolyte salt is then added therein until the concentrationof lithium hexafluorophosphate in the electrolyte is 1 mol/L.

Preparation of Lithium-ion Secondary Battery

The negative electrode, the separator, and the positive electrode arestacked in such a manner that the separator is placed between thepositive electrode and the negative electrode, and then wound into asquare bare cell having a thickness of 8 mm, a width of 60 mm and alength of 130 mm. The bare cell is packed into an aluminum foil pouch,vacuum-baked at 75° C. under vacuum for 10 hours, filled withnon-aqueous electrolyte, and stood for 24 hours after beingvacuum-sealed. Then, the battery is charged to a voltage of 3.75V with aconstant current of 0.05C, followed by being charged with a current of0.3C to a constant voltage of 4.2V until the current fell to 0.05C, thenthe battery is discharged to a voltage of 2.8V with a constant currentof 0.3C, and finally charged with a constant current of 1C to a voltageof 3.8V, so as to finish the preparation of the lithium-ion secondarybattery.

Embodiments

The lithium secondary batteries are prepared by the above method, theinorganic layers having different thicknesses and porosities aredeposited on the positive electrode active material layers of thepositive electrode by the above vapor deposition method.

The specific process parameters are as follows: the target material isaluminum metal, the vacuum degree of the deposition chamber is lowerthan 1×10⁻³ Pa, the heating evaporation current is 190 A when the targetmaterial is Al, the oxygen volume flow is 300 seem, the plasma power isapproximately 300 W, and the processing time is 5 min.

The specific parameters of the obtained positive electrode are shown inTable 1.

Aluminum oxide powder and the binder of polyvinylidene fluoride (bindercontent of 50%) are mixed and coated on the surface of the positiveelectrode active material layer. Due to the process limitations, theminimum thickness of coating that can be achieved is only 2000 nm, i.e.,the positive electrode D6 is obtained.

TABLE 1 Positive Electrode Active Material Inorganic Layer Layer AverageMass per Unit Compaction Pore Area of The Active Density ThicknessDiameter Inorganic Layer No. Material g/cm³ Porosity % Material nmPorosity % nm g/m² S1 NCM811 3.5 20 Aluminum 20 20 3.3 0.08 oxide S2NCM811 3.5 20 Aluminum 50 30 10.2 0.18 oxide S3 NCM811 3.5 20 Aluminum100 30 28.4 0.35 oxide S4 NCM811 3.5 20 Aluminum 250 30 43.1 0.88 oxideS5 NCM811 3.5 20 Aluminum 500 30 50.7 1.26 oxide S6 NCM811 3.5 20Silicon oxide 1000 30 61.5 2.52 S7 NCM811 3.5 20 Aluminum 1650 30 72.44.16 oxide/Silicon oxide composite S8 NCM811 3.5 20 AlO(OH) 2000 30 80.25.04 S9 NCM811 3.5 20 Silicon oxide 500 10 62.3 2.07 S10 NCM811 3.5 20Silicon oxide 500 40 57.4 0.92 S11 NCM811 3.5 20 Silicon oxide 500 5043.2 0.65 S12 NCM811 3.5 20 Silicon oxide 500 60 37.1 0.41 S13 NCM8113.5 10 Aluminum 500 20 60.2 1.65 oxide S14 NCM811 3.5 40 Aluminum 500 2062.3 1.62 oxide S15 NCM811 3.5 5 Aluminum 500 20 58.3 1.70 oxide S16NCM811 3.5 45 Aluminum 500 20 64.9 1.64 oxide S17 NCM811 3.5 20 Aluminum500 60 60.8 0.84 oxide D1 NCM811 3.5 20 / / / / / D2 NCM811 3.5 20Aluminum 10 30 3.2 0.04 oxide D3 NCM811 3.5 20 Aluminum 2250 30 86.25.67 oxide D4 NCM811 3.5 20 Silicon oxide 500 5 67.8 2.32 D5 NCM811 3.520 Silicon oxide 500 65 35.7 0.32 D6 NCM811 3.5 20 Aluminum 2000 60 5005.46 oxide + Binder In Table 1, “/” indicates that the structure or theparameter is non-existent.

Finally, the measurement procedures and measurement results with respectto the lithium-ion secondary batteries are described below.

(1) Test for Capacitance of Lithium-Ion Secondary Battery

In an incubator with a constant temperature of 25° C., the battery wascharged with a constant current of 1C to reach a voltage of 4.2V, andthe charge was kept at voltage of 4.2 V until the current fell to 0.05C.Then, the battery was discharged with a constant current of 1C to reacha voltage of 2.8V. The obtained discharge capacitance was thecapacitance of the battery.

(2) Test for Capacity Retention Rate of Cycling at High Temperature ofLithium-Ion Secondary Battery Under High Cut-Off Voltage

In an incubator with a constant temperature of 60° C., the battery wascharged with a constant current of 1C to reach a voltage of 4.3V, andthen the charge was kept at voltage of 4.3V until a current was droppedto 0.05C. Then, the battery was discharged with a constant current of 1Cto reach a voltage of 2.8V. The obtained discharge capacity was abattery capacity C1 of the first cycle. This charge-discharge cycle wasrepeated 1000 times to obtain a battery capacity Cn of the 1000^(th)cycle.

The capacity retention rate of the 1000^(th) cycle=Cn/C1×100%.

(3) Test for High Temperature Storage and Gas Generation Under HighCut-Off Voltage

Five batteries were selected respectively from the batteries withpositive electrode S1 and D1, and charged to a voltage higher than 4.3 Vwith a constant current of 0.3C at a room temperature and furthercharged at a constant voltage of 4.3 V until the current fell to 0.05Cso that the batteries were kept in a full charge state of 4.3 V. Aninternal pressure of each of the full-charged batteries was measured andrecorded as P₀ before the storage. The batteries then were placed in anoven at 80° C. for 15 days and then taken out. After the batteries werecooled for 1 h, the internal pressure of each of the full-chargedbatteries was recorded as P_(n)

According to a formula: ΔP=P_(n)−P₀, a pressure change value before andafter the storage of the batteries can be calculated.

The obtained experimental results are shown in Table 2:

TABLE 2 Capacity Retention Rate after Gas Production after CapacityCycles of High Temperature High Temperature No. (mAh) (%) Storage (MPa)S1 2104.2 90.2 0.37 S2 2099.9 92.9 0.31 S3 2118.4 94.6 0.29 S4 2081.395.5 0.13 S5 2070.5 95.1 0.15 S6 2072.9 94.9 0.14 S7 2083.1 94.7 0.12 S81962.1 91.1 0.15 S9 2063.8 92.3 0.11 S10 2098.0 95.9 0.17 S11 2062.494.6 0.24 S12 2102.9 91.9 0.27 S13 2008.1 93.6 0.12 S14 2053.2 91.3 0.14S15 1964.4 91.7 0.26 S16 2073.2 92.7 0.17 S17 2069.4 90.7 0.28 D1 2048.784.3 0.54 D2 2093.3 87.1 0.35 D3 1831.7 85.5 0.31 D4 2087.7 88.6 0.27 D52008.1 89.2 0.12 D6 2070.5 83.6 0.29

It can be seen from the experimental results of the above embodimentsthat, the high temperature cycle performance and high temperaturestorage performance of the lithium-ion battery can be significantlyenhanced by providing the inorganic layer containing no binder andhaving a specific thickness and a specific porosity on the positiveelectrode active material layer. When the thickness of the inorganiclayer is too great or too small, or the porosity of the inorganic layeris too large or too small, the effect of improving the high temperaturecycle performance and the high temperature storage performance islimited or even deteriorated.

The present application is disclosed in preferred embodiments as abovedescription. However, these preferred embodiments are not intended tolimit the Claims. Any skilled in the art may make changes andmodifications without departing from the concept of the presentdisclosure. The protection scope of the present disclosure shall bebased on the scope defined by the Claims.

What is claimed is:
 1. A positive electrode comprising a currentcollector; a positive electrode active material layer arranged on atleast one surface of the current collector, and the positive electrodeactive material layer contains a positive electrode active material,wherein an inorganic layer having a thickness of 20 nm to 2000 nmarranged on a surface of at least one positive electrode active materiallayer away from the current collector, wherein the inorganic layer is aporous dielectric layer containing no binder, and the inorganic layerhas a porosity of 10%˜60%.
 2. The positive electrode according to claim1, wherein the thickness of the inorganic layer is in a range of 50 nmto 1500 nm, preferably in a range of 100 nm to 1000 nm, and morepreferably in a range of 150 nm to 500 nm.
 3. The positive electrodeaccording to claim 1, wherein the inorganic layer is prepared by vapordeposition method, the vapor deposition method preferably comprising atleast one of atomic layer deposition, chemical vapor deposition,physical vapor deposition or thermal evaporation deposition.
 4. Thepositive electrode according to claim 1, wherein the inorganic layerdisposed on the surface of the positive electrode active material layeras well as inner walls of pores on the surface of the positive electrodeactive material layer, and preferably, a ratio of a depth of theinorganic layer in the pores of the positive electrode active materiallayer to a thickness of the positive electrode active material layer isin a range from 1/1000 to 1/10.
 5. The positive electrode according toclaim 1, wherein the inorganic layer has a porous structure formed bystacked nanoclusters, pores for ion transmission formed between thenanoclusters have an average pore diameter in a range from 0.1 nm to 100nm, and preferably, the nanoclusters have an average particle size in arange from 2 nm to 500 nm.
 6. The positive electrode according to claim1, wherein the inorganic layer has a mass per unit area in a range from0.03 g/m² to 7.0 g/m².
 7. The positive electrode according to claim 1,wherein the porosity of the inorganic layer is higher than a porosity ofthe positive electrode active material layer.
 8. The positive electrodeaccording to claim 1, wherein the inorganic layer further contains amaterial selected from a group consisting of aluminum oxide, AlO(OH),aluminum nitride, aluminum fluoride, silicon oxide, silicon nitride,silicon fluoride, titanium oxide, titanium nitride, titanium fluoride,zinc oxide, zinc nitride, zinc fluoride, magnesium oxide, magnesiumnitride, magnesium fluoride, zirconium oxide, zirconium nitride,zirconium fluoride, calcium oxide, calcium nitride, calcium fluoride,barium oxide, barium nitride, barium fluoride, or combinations thereof.9. The positive electrode according to claim 1, wherein the positiveelectrode active material is selected from a group consisting ofLi_(z)Co_(1−(x+y))Ni_(x)Mn_(y)O₂, LiCoO₂, LiNiO₂, LiCo_(a)Ni_(1−a)O₂,LiVPO₄, Li₂MnO₃, LiMn_(2−b)M_(b)O₄, LiFe_(c)Mn_(1−c)PO₄, or combinationsthereof, in which 0<x<1, 0<y<1, 0.9<z<12, 0<a<1, 0≤b<2, 0<c≤1, and M isselected from a group consisting of Li, Al, Mg, Zn, Cr, La, Nd, Ti, Sn,V, or combinations thereof.
 10. The positive electrode according toclaim 1, wherein particles of the positive electrode active material areprovided with a coating layer, and the coating layer contains at leastone of Al₂O₃, CaO, SiO₂, TiO₂, B₂O₃, MgO, ZrO₂ or lithium transitionmetal phosphates, and preferably, the coating layer has a thickness in arange from 5 nm to 500 nm.
 11. A method for preparing a positiveelectrode, comprising at least following steps: preparing a slurry bymixing a positive electrode active material, a binder and a conductiveagent; coating on at least one surface of a current collector with theslurry and drying the slurry to form a positive electrode activematerial layer; and preparing an inorganic layer having a thickness of20 nm to 2000 nm on a surface of the at least one positive electrodeactive material layer away from the current collector by vapordeposition method, wherein the inorganic layer is a porous dielectriclayer containing no binder, and the inorganic layer has a porosity of10%˜60%.
 12. The method for preparing a positive electrode according toclaim 11, wherein the vapor deposition method comprises at least one ofatomic layer deposition, chemical vapor deposition, physical vapordeposition, or thermal evaporation deposition.
 13. An electrochemicaldevice, comprising a positive electrode, a negative electrode, aseparator and an electrolyte, wherein the positive electrode is thepositive electrode according to claim
 1. 14. The electrochemical deviceaccording to claim 13, wherein the electrochemical device is alithium-ion secondary battery, and the thickness of the inorganic layerhas no change when the lithium-ion secondary battery reaches a voltageof at least 5 V.